This invention relates to the formation of specification char from lignite in general and more particularly to a new and improved process for utilizing an existing furnace of the Herreshoff type or the like to produce specification char.
It is known in prior art types of furnace construction to be able to produce charcoal from materials that were formerly wasted or burned such as bark, shavings, chips, slabs, trim and sawdust and also from agricultural wastes such as nutshells, hulls, fruit pits and bagasse in a Herreshoff Carbonizer Furnace. This type of furnace is capable of carbonizing these materials thoroughly and uniformly despite their high insulating properties and small particle sizes, because the furnace stirs the materials continuously in thin layers over a large flat surface area moving the materials from one hearth to an adjacent hearth until the material has been carbonized to the desired degree.
Furnances of the Herreshoff type generally utilized temperatures in the first or drying hearth ranging from 800.degree. to 1400.degree. Fahrenheit with temperatures in the last hearth ranging from 1000.degree. to 1400.degree. Fahrenheit averaging approximately 1200.degree. F. Furnaces of this type are shown in the U.S. Pat. No. 3,379,622 issued Apr. 23, 1968, to C. F. Von Dreusche, Jr. While satisfactory for producing char from wood and particles similar thereto generally weighing in the range of 18 to 20 lbs./cubic foot, these furnaces were not satisfactory for manufacturing lignite char weighing approximately 40 lbs./cubic foot and having a much higher volatile content. When attempting to use the Herreshoff furnace with lignite, it was found that by utilizing the normal temperatures, as for example in the first hearth of 800.degree. to 1400.degree. F, the volatile products that were driven off were very large in volume and comprised a mixture of heavy and light hydro-carbons which were difficult to handle and condensed in the production system where it was undesirable. As a result of this condensation, the explosion potential of this gassy condition, the potential for agglomeration of the lignite, and the odor given off by the hydro-carbons, it was felt that the Herreshoff furnace was undesirable for the manufacture of lignite char when operated according to established multi-hearth procedures well known to the art.
Other types of furnaces have been used for the manufacture of charcoal such as the Lurgi furnace which has been in existence since the early 1900's and was used by the Germans to extract hydro-carbons for the manufacture of gasoline and kerosene during the second World War. The Lurgi process is a "low temperature" process and takes the off gases and scrubs them whereupon they are condensed and the tars and pitches in the gases are extracted and the gases are used for other purposes. Problems were also encountered in using the Lurgi type furnaces since a certain percentage of the light hydro-carbons are emitted during production of the char along with the thirty-five percent moisture that first comes off of the lignite resulting in obnoxious odors emitting into the surrounding neighborhoods. The lower temperatures at which the furnaces operate are not sufficient to burn off the hydro-carbons allowing them to go off in the form of steam and light volatiles that produce the unpleasant odor. From experimentation, it was finally determined that the problem in using the prior art type furnaces resulted from the fact that lignite being a lesser form of coal and containing approximately thirty-five percent water, was a much more complex material to carbonize in the furnace since the volatile content of lignite was much more complex compared to the wood previously carbonized in the furnaces. In addition, the lignite molecules were much larger than wood resulting in their hydro-carbons being much bigger and heavier meaning that if they were carbonized at the lower temperatures utilized in the prior art furnaces, a greater pollution problem would occur plus explosion problems which were potentially available unless the various exhaust systems were kept clean. These problems resulted from the fact that the hydro-carbons of lignite were found to condense out in the various parts of the system where it was highly undesirable for the condensation to occur.
It is known in the art of carbonizing lignite and other materials to manufacture briquettes which are used in a metallurgical process where high compressive strength is required that the process is one in which the coal tar pitch volatiles are retained and/or recovered and become an important part of the binding process directly in the formulation of the briquette. Briquettes of this type are different from the briquette that the applicant is attempting to manufacture with the applicant's briquette being utilized for sale to a home consumer to be used for backyard cooking where high compressive strength is not required in the briquette. Such prior art briquettes manufactured for metallurgical processes are typified in the following U.S. patents:
______________________________________ Pat. No. Issue Date Inventor ______________________________________ 1,443,359 January 30, 1923 C. I. Gillstrap 3,671,402 June 20, 1972 W. Wenzell et al 3,758,385 September 11, 1973 K. O. P. Fischer 3,980,447 September 14, 1976 F. Franke et al 3,026,252 March 20, 1962 W. Muschenborn et al 3,414,480 December 3, 1968 H. H. Stotler et al 3,051,629 August 28, 1962 E. Gorin et al 3,140,241 July 7, 1964 J. Work et al 3,184,293 May 18, 1965 J. Work et al 3,316,155 April 25, 1967 M. O. Holowaty et al 4,002,534 January 11, 1977 R. Rammler et al ______________________________________
In the carbonization of lignite it is known to produce charcoal briquettes which are clean burning and are suitable for use in cooking of food over an open fire. The processes for manufacturing these lignite barbeque briquettes are typified in the following two patents:
______________________________________ Pat. No. Issue Date Inventor ______________________________________ 3,026,189 March 20,1962 A. B. Hall 3,062,629 November 6, 1962 S. V. Margolin et al ______________________________________
It is noted that in the Hall process (3,026,189) there is utilized a rotary kiln which is fired internally with natural gas and an air mixture which provides for substantial de-volatilization at temperatures ranging from 1150.degree. to 1450.degree. with a retention time of 45 to 48 minutes. Such a process by its very nature does not provide for any meaningful control of the degree of de-volatilization of the lignite as does the applicant's invention as will be described more fully hereinafter and in addition the Hall process is not self-sustaining since the rotary kiln requires natural gas as a fuel in order to carbonize the lignite.
The Margolin process (3,062,629) deals with the beforementioned Lurgi process and furnace of which there are only two in the United States both of them being owned by the assignee of the subject invention. The Margolin process is a low temperature process operating in the range of 500.degree. to 550.degree. C or 932.degree. to 1022.degree. F.